Recent clinical investigations of pituitary function suggest that pituitary adenomata may result from pituitary stimulation by releasing factors of hypothalamic or ectopic origin. Feedback on many secretory cells by their own secretory products is knows to modulate their response. In preliminary studies of a strain of rat pituitary tumor cells (GH3) which secrete both prolactin (PRL) and growth hormone (H) we have observed: (1) specific inhibition of hormone synthesis and secretion in tissue culture when medium hormone nears 1 Mug/ml; (2) stable hormone synthesis and secretion in the first 10-12 hours of perifusion; (3) a spontaneous increase in the rate of PRL synthesis and secretion after 10-12 hours and a separate spontaneous increase in the rate of GH synthesis and secretion at 20-28 hours of perifusion of the same cells; (4) continuing increase in the rate of hormone synthesis and release for 80 hours of perifusion; (5) complete and specific inhibition of the spontaneous increase of PRL synthesis and secretion by adding 600 ng/ml rPRL to perifusion medium; and (6) a suggestion of similar specific inhibition of GH, but not PRL, by 600 ng/ml of rGH. This application proposes (1) in the GH(3) tumor cell to: (a) characterize maximum synthesis and secretory capacity for each hormone, (b) define factors such as specific hormone receptors, and dependency upon DNA, RNA, and/or protein synthesis, which influence the spontaneous increase in hormone release, (c) investigate the specificity of the feedback response using placental lactogen (PL); and (2) in the normal rat to: (a) pursue a qualitatively similar phenomenon in intact pituitaries as well as in somatotrophs and lactotrophs isolated by density gradient centrifugation seeking quantitative differences in responsivity, and (b) compare synthesis and release responses to hypothalami or hypothalamic extracts at various ambient hormone levels. Proposed experiments are performed in tissue culture, pituitary and cell perifusion, and in vivo. Radioimmunoassay follows general hormone release and specific immunoprecipitation differentially follows release of stored hormone and synthesis and release of newly synthesized hormone. This work will enhance understanding of the regulation of normal pituitary hormone control mechanisms, may contribute to understanding the genesis of pituitary adenomate and will consider possible physiologic interrelations between GH, PRL and PL.